22

Chapter II

Experimental Techniques

2.1 Introduction

This Chapter deals with brief description of the equipments with their relevant

details and specifications used in the experiments carried out and various characterization

techniques for the measurements employed in this thesis. The present work involves the

following stages:

[1] The materials: Polymers studied and their chemical structure.

[2] Brief description of 15 MV Pelletron Accelerator at Inter University, Accelerator

Centre (IUAC), New Delhi and heavy ion beams from it.

[3] Material science chamber for irradiation

[4] Positron Source.

[5] Positron Annihilation Lifetime Measurements.

[6] Ultra Violet (UV-Vis.) Spectroscopy

[7] Fourier Transform Infra-Red (FTIR) Spectroscopy.

[8] X-Ray Diffraction (XRD) study

[9] Dielectric constant measurements

2.2 The materials

The polymeric materials investigated in the present study are:

(i) Makrofol-KG Polycarbonate

(ii) Makrofol-N Polycarbonate

(iii) Lexan Polycarbontae

(iv) Polyethersulphone (PES)

(v) Polyamide Nylon-6

(vi) Polyamide Nylon-6, 6

23

(vii) Polytetrafluoroethylene (PTFE)

(viii) Polyethylene terephthalate (PET)

(x) Polypropylene (PP)

(xi) Polymethyl methacrylate (PMMA)

(xii) Polystyrene (PS)

(xiii) Polyvinylidene fluoride (PVDF)

(xiv) Low Density Polyethylene (LDPE)

(xv) Polyethylene Oxide –salt (PEO-salt)

(xvi) Polyanilinegraphite (PANI-GRP) pellets

2.2.1 Makrofol-KG, Makrofol-N and Lexan Polycarbonates

Makrofol polycarbonates manufactured by a casting process in the form of thin

sheets were obtained from Bayer AG, Lever Kussen, West Germany. (C16 H14 O3) as

Lexan polycarbonate was manufactured by General Electric Co. of U. S. A.

Polycarbonate (PC) or specially polycarbonate of bisphenol- A, is an amorphous polymer

with attractive engineering properties including high impact strength, low moisture

absorption, good dimensional stability and high light transmission. Polycarbonate gets its

name from the carbonate groups in its backbone chain. However, these polycarbonates

have different type of behavior. Different types of Makrofol polycarbonates such as

Makrofol-KG, KL, E, and N etc. are produced by different manufacturing processes and

are expected to behave in differently [1]. Out of these Makrofol-KG contains a light

amount of a colour dye while Makrofol-N is a trade name of yellow polycarbonate.

24

Makrofol polycarbonate plastics (much useful as heavy ions as well as fission

track detectors) were originally used as insulators in electrical devices. Makrofol-KG, KL

and N are not sensitive to particles having Z > 2 and Z > 8 respectively [2]. Makrofol-

KG, KL and N are not sensitive to -particles and other lighter charged particles. The

shape of tracks produced by heavy ions and fission fragments are needle like with a slight

spread towards its tail. The chemical structure of Polycarbonate (PC) is

2.2.2 Polyethersulphone (PES)

Aromatic polymers such as Polyethersulphone (PES) are finding extensive use in

electronics, in particular in sensor applications. The physical properties of these films

may be tailored as has been shown that ion irradiation improves the sensor properties of

PES films[3-5]. PES flat films were procured from Good Fellow, Cambridge Ltd.,

England (U.K.) and have the structure:

2.2.3 Polyamide Nylon– 6 (PN-6) and Polyamide Nylon-6,6 (PN-6,6) Polymers

These polymers were obtained from Good fellow, Cambridge Limited, England.

The polyamides are a family of thermoplastics e.g. Polyamide Nylon–6, Nylon-6,6 and

Nylon-610 which are among the toughest engineering plastics with high vibration–

damping capacity, abrasion resistance, inherent lubricity and high load capacity for high

speed bearings. They have a low coefficient of friction and good flexibility. Pigment –

stabilized types are not affected by ultraviolet radiation and have good chemical

resistance. Polyamides are used extensively as high performance plastics materials

C

CH3

CH3

O C

O

O

n

25

because of their unique combination of superior mechanical, electrical, chemical and

thermal properties. Applications include bearings, electrical insulators, gears, wheels,

screw fasteners, cams, latches, fuel lines and rotary seals. The molecular structure of

Polyamide Nylon-6, polymer used for present study is

C

O

CH2 N

H

5

2.2.4 Polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) has been classified for many years as a polymer

that undergoes main chain scission by irradiation [6]. In some recent papers [7-10] it is

described that PTFE is cross linked by ionizing radiation in an oxygen-free atmosphere at

a temperature above its melting point. Therefore, the effect of irradiation on high

crystalline PTFE (at room temperature) has to be smaller than an amorphous PTFE (in the

melt). Additionally further qualitative changes will be expected by irradiation of molten

PTFE. Lappan et al [11] have also studied the behavior of PTFE on such special

irradiation conditions. PTFE Polymer was procured from Good fellow, Cambridge Ltd.

England (UK). The chemical structure of PTFE is:

2.2.5 Polyethylene terephthalate (PET)

Polyethylene terephthalate (PET) is a polyester having a high melting point due to

the presence of aromatic ring and a very good mechanical strength. It is semi crystalline

in nature and is resistant to heat and moisture and virtually unattacked by many

chemicals. It has extensive use in textile fibers. The changes brought about in physico-

chemical properties of PET as a result of exposure to light as well as heavy ions have

been a subject of investigation for many years. Mishra et al [12] studied the changes in its

26

thermal and chemical properties by exposing it to swift light ions, protons. Steckenreiter

et al [13] did an in depth study of chemical modification of PET exposed to Swift Heavy

Ions of molybdenum and krypton. Recently Liu et al [14] and Zhu et al [15] have

extended its study by exposing it to heavy ions of argon, krypton, xenon and uranium

having energy in the range of 1.4-2.7 GeV. Singh et al [16] too have also recently

reported a study on the electrical and structural properties of PET films modified by 50

MeV lithium ions PET Polymer was procured from Good fellow, Cambridge Ltd.

England (UK). The chemical structure of PET is.

2.2.6 Polypropylene (PP)

Polypropylene is a vinyl polymer and is similar to polyethylene only that on every

other carbon atom in the backbone chain has a methyl group attached to it. Polypropylene

can be made from the polymerization of monomer propylene. Polypropylene is the

lightest known industrial polymer, and it has high strength-to- weight ratio. Being highly

crystalline, it exhibits high stiffness, hardness and tensile strength and has excellent

mechanical and dielectric properties. PP in the form of flat films was procured from Good

Fellow, Cambridge Ltd. England (U.K.). The chemical structure of PP is.

2.2.7 Polymethyle methacrylate (PMMA)

Molecular weight of a polymer is of major importance in its synthesis and

application. Interesting and valuable mechanical properties that are uniquely associated

with polymeric materials are a consequence of their high molecular weights [17].

Polymethyl methacrylate (PMMA), is completely amorphous but it has high strength and

excellent dimensional stability due to its rigid polymer chains. PMMA has exponential

27

optical clarity, very good weather ability, and impact resistance. It is an excellent material

which is easy to structure and has the desired optical properties [18]. PMMA, known as a

positive photo-resist for its degradation upon irradiation, has been the subject of

investigations in radiolysis than many other polymers. This was partly due to a growing

interest in the application of PMMA in ion beam lithography, in the semiconductor

industry [19]. Besides wide range utilization of PMMA polymer such as its extensive use

in expanding optical networks in the field of telecommunication, PMMA polymers have

many application such as diffusers, indoor and outdoor lighting, lenses, and contact

lenses. PMMA Polymer was procured from Good fellow, Cambridge Ltd. England (UK).

The chemical structure of PMMA is.

C C

H CH3

C

H

HCH3H

O

CH3

O

CH3

O

CH3 H

H

C

CH3

C

O

2.2. 8 Polystyrene (PS)

Polystyrene is polymer of having wide industrial applications. Its radiation

chemistry has been extensively studied. It is the most radiation resistant of the polymers

and hence occupies a unique position in the study of radiation effects. Polystyrene sheets

were obtained from Good fellow, Cambridge Limited, England. The molecular structure

of polystyrene is:

2.2.9 Polyvinylidene fluoride (PVDF)

PVDF is a specialty plastic material in the fluoropolymer family. it is used

generally in applications requiring the highest purity, strength, and resistance to solvents,

acids, bases and heat and low smoke generation during a fire event. Compared to other

fluoropolymers, it has an easier melt process because of its relatively low melting point. It

28

has a relatively low density and low cost compared to the other fluoropolymers. It is

available as piping products, sheet, tubing, films, plate and an insulator for premium wire.

It can be injected, molded or welded and is commonly used in the chemical,

semiconductor, medical and defense industries, as well as in lithium ion batteries. PVDF

paints have extremely good gloss and color retention, and they are in use on many

prominent buildings around the world, e.g. the Petronas Towers in Malaysia and Taipei

101 in Taiwan, as well as on commercial and residential metal roofing. PVDF membranes

are used for western blots for immobilization of proteins, due to its non-specific affinity

for amino acids The (PVDF) in the form of flat films was procured from Good Fellow,

Cambridge Ltd. England (U.K.). The chemical structure of PVDF is

2.2.10 Low Density Polyethylene (LDPE)

Polyethylene is a vinyl polymer, made from the monomer ethylene. Vinyl

polymers are the polymers made from vinyl monomers i.e. small molecules containing

carbon-carbon double bonds.

So, a molecule of polyethylene is nothing more than a long chain of carbon atoms,

with two hydrogen atoms attached to each carbon, like,

Sometimes some of the carbon atoms, instead of having hydrogen attached to

them will have long chains of polyethylene attached to them. Polyethylene is of two

types (i) Branched polyethylene, known as low density polyethylene or (LDPE).(ii)

http://en.wikipedia.org/wiki/Wire_wraphttp://en.wikipedia.org/wiki/Western_blot

29

Linear polyethylene, known as high density polyethylene or (HDPE); as shown in

Figure 2.1.

Figure 2.1: (a) Branched polyethylene or (LDPE) (b) Linear polyethylene or HDPE

Polyethylene is one of the common polymers utilized in various fields; engineering,

medical, and agricultural and even our daily life. LDPE Polymer was procured from

Good fellow, Cambridge Ltd. England (UK).

2.2.11 Polyethylene Oxide –salt (PEO-salt)

Solution-cast films each of total mass 3g of PEO (BDH, England) and of average

molecular weight 600 Kg/mol complexes with NH4ClO4 (Fluka AG, 99.5% purity) were

prepared in salt concentration of 17% and 19%. Pure PEO is non conducting while its

complex PEO(1-x) (NH4ClO4)x with weight fraction x = 17%, 19% is an ion conducting

polymer.

2.2.12 Polyaniline-graphite (PANI-GRP) pellet

Synthesis and investigation of electro physical properties of polymer composites

based an polyconjugated matrices and graphitized carbon species, is currently a very

30

popular topic in the field of material science. PANI-graphite polymer composites are

regarded as promising material for use in lithium batteries, super capacitors, actuators and

sensors etc. Conjugated polymer PANI was prepared as a product of oxidative

polymerization of aniline. The reaction temperature was in the range 0-10oC. Polyaniline

chloride was filtered, washed with double distilled water and dried at 450 C for 24 hrs and

then treated with 3% aqueous ammonia. Polymer composite based on polyaniline (PANI)

and graphite were prepared using powder technology whereby the polymer and filler

powders were mixed at room temperature in 1:2 ratio.

2.3. Irradiation of Polymers by Swift Heavy Ion Beams

2.3.1. 15 UD Pelletron Accelerator at Inter University Accelerator Centre

(IUAC), New Delhi.

A schematic diagram of IUAC Pelletron accelerator is shown in Figure 2.2. The

IUAC Pelletron accelerator is 15 UD tandem Van de Graff electrostatic accelerator [20].

It is capable of accelerating any ion from proton to uranium (except the inert gases) in the

energy range from a few tens of MeV to a few hundreds of MeV, depending on the ion

species. The accelerator is installed in vertical geometry in a stainless steel tank which is

26.5 meter high and has 5.5 meter diameter. In the middle of the tank there is a high

voltage terminal which can hold potential from 4 to 16 MV. The terminal is connected to

the tank vertically with ceramic titanium accelerating tubes. The tank is filled with high

dielectric constant SF6 gas at 6-7 atmospheric pressure to insulate the high voltage

terminal from the tank wall. A potential gradient is maintained through the accelerating

tubes from the ground potential, and from the terminal to the ground potential at the

bottom of the tank. Negative ions of suitable energy from source of negative ions by

Cesium Sputtering (SNICS) ion source are injected into the accelerator and are

accelerated towards the positive terminal. In the first stage of acceleration, the singly

charged negative ions from the ion source are accelerated from ground potential to the

terminal at high positive potential V. The energy gained in the process is eV. The beam is

then made to pass through a stripper foil where the ions are stripped off the electrons

31

thereby making them positive ions. The average charge of the ion depends upon the type

of the ion and the terminal voltage.

Figure 2.1: A schematic diagram of NSC Pelletron Accelerator

32

If q is the charge state on the positive ions after passing through the stripper foil, the

energy gained by accelerating it from the terminal to the ground potential should be qeV.

Thus after passing through the two stages of the acceleration, the final energy of the ion

in electron volts is given by

E = (q+1) V (2.1)

These high-energy ions are then passed through the analyzing magnet and energy slit

which selects the particular ions of the desired energy. The beams of ions are then

directed towards the desired experimental area with the help of a seven port-switching

magnet.

2.3.2. Chamber for high fluence irradiation at IUAC

This high vacuum chamber (38 cm diameter) has a facility for temperature

controlled (liquid cooled) multiple sample holders having provision for linear movement

of 120 mm and a rotation of 3600 shown in photograph 2.1. A vacuum of 10

-7 m bar is

maintained by using a diffusion pumping system filled with a LN2 trap. A remote

controlled target holder can be positioned perpendicular to the beam line for irradiation.

Various samples can be irradiated in an experiment using bellow-sealed linear movement

of the holder by 140mm. Material science beam line is used for ion fluence up to 1013

ions/cm2. For irradiation one has to observe whether ion beam is falling at the desired

place on the quartz or not, only after that the beam is allowed to fall on the samples to be

irradiated. The rectangular ladder used to fix up seven samples contains four faces and

auto control switches can change its position. A CCTV camera was also attached to one

of the ports of chamber for viewing the sample position. XY scanner can scan the beam

on the target for uniform irradiation.

33

Photograph 2.1: General Purpose Scattering Chamber installed at 150 beam line

at Inter University, Accelerator Center, New Delhi

34

2.4 Characterization Techniques

2.4.1 Positron Annihilation Spectroscopy

2.4.1.1 Positron

Positron was discovered by Anderson [21] on 2nd

August, 1932 from the length of

the tracks in the Wilson cloud chamber. Anderson concluded that he had observed the

particle with same mass as that of electron but of opposite charge i.e. +e. Hence Positron

is the anti particle of the electron.

2.4.1.2 Positron Annihilation

The damage caused by the passage of energetic ion modifies the free volume

properties of the polymeric material. The concept of free volume has significant

importance for the gas permeation properties of polymeric membranes as well as for other

related subjects of polymer science. The positron annihilation lifetime spectroscopy is

capable of probing free volumes directly. The atomic scale free volume holes are detected

on the basis that the positronium (Ps) atoms are formed and localized in the free volume

holes [22]. The ortho-positronium (o-Ps) lifetime has a strong correlation with the size of

the free volume.

Now a days, Positron annihilation has become established as a useful tool in the

field of material science and is successfully applied for the investigation of defect

structures present in metals, alloys and technologically relevant materials such as

polymers. Positron-electron pair is unstable and annihilates by emitting two -photons of

energy 511 KeV each in opposite direction, since the linear momentum is to be

conserved.

35

Positron annihilation is undertaken to the study of Fermi surface of the metals

and alloys and also it has been found that positron annihilation is quite sensitive to the

lattice defects and is a common technique used in the study of lattice defects, phase

transitions and liquid alloys. Positron electron pair can also be formed as a quasi-

stationary state called positronium. It is analogues to the hydrogen atom. Positronium is

found in the Para state called Para-positronium (p-Ps) and ortho state called ortho-

positronium (o-Ps). Para-Ps will decay through two - photons and the ortho-Ps will

decay by three -photons. Ps is generally not formed in metals. Positron annihilation

lifetime spectroscopy provides direct information about the dimension, content and hole

size distribution of the free volume in polymers. The Positron annihilation technique has

also made its entry in the field of semiconductor technology and the positron annihilation

measurement techniques (lifetime, Doppler-broadening and angular correlation) integrate

a large number (often106) of annihilation events. Hence today this technique has become

established as a useful tool in material science and has been successfully applied for the

investigation of defects structures present in technologically relevant materials like

polymers.

2.4.2 Positron Annihilation Lifetime Spectroscopy (PALS)

The Positron annihilation Spectroscopy is one of the most important tools for the

study of defect in solids. Almost all experiments use the properties of the two-gamma

annihilation reaction.

e+ + e

- → 2 γ (2.2)

when a positron with an energy of few hundreds of KeV, e.g. from nuclear decay from

radio-isotopes such as 22

Na enters molecular solids, it interacts with the molecules

through elastic collision processes. It reaches the thermal energy in a few picoseconds by

a succession of ionizing collisions, electron hole excitation and phonon interactions. For

the period of thermalization and at nearly thermalized stage, a positron captures an

electron from the surrounding medium and forms an atom of positronium (Ps). Thus the

36

bound state of positron-electron pair is formed similar to a hydrogen atom. Therefore,

during its lifetime, the positron may exist in both positron and Ps states in molecular

solids. The polymeric materials have local free volumes which have the radius of few Å.

These are the favorable sites where the positron and Ps atoms are localized before

annihilation [23, 24]. It is schematically shown in the Figure 2.2. In the polymeric

material, the positron has following two possible states at the time of annihilation

(i) Free (delocalized) and /or localized positron state

(ii) Free and /or localized Ps state.

The localization sites are free volume holes which are more favorable sites than

the bulk for positrons and Ps. The Positron Annihilation Spectroscopy probes only free

volume regions and is not interfered by the bulk properties of the polymeric material [26].

The ground state of Ps atom has two spin states: (i) Singlet, 1 1So state, or an anti parallel

spin state, called para-positronium. (p-Ps) (ii) triplet, 1 3So state or a parallel spin state,

called ortho-positronium (o-Ps). The energy splitting is only 8.4x10-4

eV, the singlet state

being the lower one. Accordingly, in the absence of ortho-para conversion, ¼ of the

positronium atoms are in the singlet state and ¾ in triplet state.

37

Figure 2.2: Schematic view of Ps localization in free volume holes in a Polymeric

material [25]

38

The para-positronium in free space annihilates with mean life of 125 ps, by

emitting 2γ-rays of 511 KeV each in opposite directions whereas o-Ps annihilates in

vacuum with mean life of 142ns by emitting 3γ-rays (continuous energy spectrum). The

o-Ps lifetime in condensed matter is considerably smaller than the vacuum value of 142

ns, because of the pick-off annihilation of positron by surrounding electrons of

appropriate spin orientations (anti-parallel one) via two-quantum emission. The lifetime

of the o-Ps confined in the local free volume of the polymers lies typically between 2 to

5x10-9

s. [24, 27-31].

The positron lifetime can be registered as a time difference between the emissions

of 1.27 MeV γ-quantum generated almost simultaneously with the positron in 22

Na

isotope which is the most commonly used positron source and one of the 0.511 MeV

annihilation γ-quanta Figure 2.3.

The lifetime spectrum of polymeric material is conventionally described by a sum

of discrete exponentials:

n

t

iieItN

)( (2.3)

where n is the number of exponential terms, Ii and λi representing the number of positrons

(intensity) and the annihilation rate respectively for the annihilation from the ith state. The

positron annihilation rate, λi is the reciprocal of positron mean lifetime, ηi. The PAL

spectrum is fitted by a finite number of component terms, n, using the computer codes

[29, 32]. For polymers n=3 is selected to fit the observed lifetime spectrum. A typical

positron lifetime spectrum in polytetrafluroethelene, commercially known as Teflon is

shown in Figure 2.4. The three components which appear in the lifetime spectrum are

attributed to the annihilation of p-Ps, free positrons (not Ps) and o-Ps. The shortest

component, η1 = 0.13±0.03 ns with intensity Ii = 7-20 %, is attributed to p-Ps. The

intermediate component, η2 =0.4-0.5 ns and the intensity I2 = 40-60 %, is attributed to the

direct annihilation of positrons. The longest component, η3 = 2-5 ns and intensity between

10-30 %, is attributed to o-Ps annihilation in free volumes. The o-Ps lifetime is found to

sensitively depend on the size of the free volume [34-35].

39

2.4.3 Positron Annihilation Spectroscopy in Polymers

Positron Annihilation Spectroscopy (PAS) has become established as a useful

tool as almost certainly the most valuable and winning technique for the direct assessment

of the free volume in polymers. PAS is capable of determining the local hole size and

free volume fractions in polymers without interfering significantly with the bulk of the

polymers. PAS has also been developed to be a quantitative probe of the free volume. It

also gives detailed information on the distribution of free volume hole size in the range

from 1 to 10 Å. The annihilation of positrons in condensed matter like polymers provides

a unique way of obtaining information about the internal structure of material. This

information is transmitted through γ-rays, emitted when the positron annihilates in the

material. The internal structure of the material may be probed by measuring three

fundamentally different quantities:

Angular correlation between emitted γ-rays,

The energy distribution of the γ-rays

The lifetime distribution of Positron.

Thus the Positron Annihilation Spectroscopy is a family of three experimental

techniques:

Angular Correlation of Annihilation Radiation (ACAR)

Doppler Broadening Spectroscopy (DBS)

Positron Annihilation Lifetime Spectroscopy (PALS)

These three methods of positron annihilation are shown in Figure 2.3. Because

energy and momentum are conserved in the annihilation process, the two γ – rays

resulting from the usual electron–positron pair annihilation each have an energy equal to

the rest-mass energy of an electron or positron (moc2 = 511 keV) ± ΔE where ΔE is an

energy shift. The two γ–rays nearly propagate in opposite directions ± an angular

deviation θ, as shown in Figure 2.3 the deviations ΔE and θ arise from the net momentum

of the annihilating positron-electron pair. However, since the positrons have only thermal

energies just prior to annihilation, the values of ΔE and θ corresponded only to the

momenta of the annihilating electrons. All these techniques have recently been applied to

polymers. In the present study PALS has been used to investigate modifications in free

volume holes in the polymers induced by the ion bombardment.

40

Figure 2.3: Three methods of positron annihilation.

41

Figure 2.4: A typical lifetime spectrum of Teflon (Polytetrafluroethelene) [36]

42

2.4.4. Nano Scale-Void detection by PAL measurements

o- Ps lifetime which is due to annihilation by pick-off process is determined by

the overlap of Ps wave function with bulk electrons of surrounding medium and the o-Ps

lifetime becomes dependent on the trap size. Thus, PAL method can used to probe the

free volume in amorphous media. The relation between free volume size and the o- Ps

decay rate, λ3, is given by Tao-Eldrup model [20,37], assuming that the Ps is trapped in a

spherical hole with radius Ro = R + R having an infinite potential barrier. This simple

model for Ps confined in a spherical box is schematically shown in Figure 2.5. Tao-

Eldrup model treats the o-Ps atom as a single scalar particle with twice the electron mass

trapped in an infinite spherical potential well in the ground state. In the central portion of

the well the o-Ps atom is assumed to have an infinite lifetime (the finite 142 ns vacuum

lifetime is ignored) and with in a distance R from the walls of the walls of the well the

o-Ps atom is assumed to have the spin-averaged Ps lifetime. The overall annihilation rate

is calculated by averaging the annihilation rate over the volume of the pore using the

square of the normalized o-Ps wave function as a weighting factor. Since lifetime is

ignored, the calculated annihilation rate is actually the pick-off annihilation rate due to

interactions with electrons in the walls of the well. Using this model the annihilation rate

due to interactions with electrons in the walls of the well. Using this model the

annihilation rate of o-Ps trapped in a pore of radius R + R is given by λ where λ =

(1+λ1+λ3)/4 is the spin averaged vaccum annihilation rate and λ1, λ3 are the singlet and

triplet vacuum annihilation rates. This model has one free parameter, R, which is

determined to be 1.66 Å by fitting to data taken in well characterized small pore materials

such as zeolites [38] and has been shown to be quite material independent.

43

Figure 2.5: A schematic diagram for a semi-empirical quantum model for Ps localized in a spherical box

with a radius R, which includes the radius of free volume hole Ro, and a uniform electron

layer with a thickness R. The ground state Ps wave fuction is schematically shown and the

annihilation rate is proportional to the overlap between the Ps and electron densities as shown

in the shades are [26].

This treatment neglects both the finite o-Ps lifetime in the central portion of the

well and the possibility that excited states in the well may be populated. For pores, 1 nm

in radius at room temperature both these effects can be ignored since typical pick-off

annihilation rates of order 0.5 ns-1

are much larger λ3 and since the energy gap between

the ground state and first excited state, of order 140 MeV, is larger as compared to kT.

Assuming that the annihilation rate of o-Ps inside the electron layer is 2 ns-1

, which is the

spin averaged annihilation rate of p-Ps and o-Ps and is also very close to annihilation rate

of Ps [26], the o-Ps lifetime as a function of free volume radius R is given by.

3 =

1

003

2

2

11

2

11

R

RSin

R

R

(2.4)

where Ro = R + R

The correlation between, η3, and free volume (spherical) is shown in Figure 2.6.

44

Figure 2.6: A correlation curve between the observed o-Ps lifetime and the volume of the free

volume holes. The solid line is the best fit using equation 1.4 with R=1.656 Å.

The data points are the measured o-Ps lifetimes in molecular systems with known

pore size [38].

45

2.4.5 Meaning of Positron Annihilation Spectroscopy (PAS)

Two types of information can be obtained from the positron annihilation

experiments. First related to the electron density at the annihilation site and the other

related to momentum distribution of the electrons. Angular correlation or Doppler

broadening experiments gives information regarding to the distribution of electron

momenta. The experiment of two gamma to three-coincidence ratio gives the information

regarding the fraction of positrons which are annihilating after positronium formation.

The same is also obtained by lifetime measurement method. The electron density

distribution is substantially affected by defects, present in the crystal and the mean

lifetime of positron is sensitive to electron density. Thus the lifetime measurement

method is commonly employed for these studies. There are mainly four reasons for the

rapid growth of Positron Annihilation Spectroscopy.

(i) Positron can provide unique information on a wide variety of problems in

condensed matter physics

(ii) Positron annihilation can be applied in the field of non-destructive testing of

the material as the information is carried out of the material by the penetrating

annihilation radiation

(iii) In recent years Positron Annihilation Spectroscopy has provided a unique

probe to study the size and number distribution of the sub nanometer cavities

(iv) Now a days, equipment is not very expensive and is commercially available.

2.4.6 Positron Sources

More than 200 positron- emitting nuclides are known of which about a dozen can

be used as sources in positron annihilation experiments [39]. A large majority of

investigations on solids by positrons has been done with 22Na or 58Co positron sources,

mainly because of their low production costs and relatively convenient half-lives. Table

2.2 lists many of the relevant properties of the commonly used isotopes [40].

The significant properties of a positron emitting nuclide are:

(a) Positron capitulate

46

(b) End point energy

(c) Half life

(d) Ease of production

The significance of the positron capitulate is trivial. The end point energy is in

two respects, the first being the positron penetration in the sample „X‟ (2.5)

X = 1/+ In I (o)/I (x) (2.5)

where X is the penetration distance in the sample, + is the absorption coefficient,

I (0) is the initial positron density I (X) is the positron density at x after a beam of

positron initial density I (0) traversed a thickness x of a given sample (and the second the

percentage of positrons annihilating in the source). One can thus expect by virtue of the

end-point energies that the most energetic positron can penetrate the sample to a depth of

about 1 mm, although some positrons will stop or return to and annihilate at or near the

surface of the sample.

Half-life of the nuclide should be large enough to be able to perform a series of

measurements with the same source. However there is no need for a half –life larger than

a few years. The energy of the start gamma should be significantly higher than that of the

annihilation gamma rays to make the recognition easy and prevent spectrum distortion.

Table 2.1 shows the end point energy, half-life and positrons per decay for several

isotopes. For lifetime or Doppler measurements, the simplest way to guide the positron

into the samples is to use a sandwiched configuration.The source should be very thin so

that only a small fraction of the positrons annihilate in the source. Radioactive material

can be deposited directly on the samples or consist of a single radioactive metal foil. In

our experiment we used the radioactive isotope 22

Na. The decay scheme of 22

Na is shown

in figure 2.2. 22

Na decays through positron emission and by electron capture to the first

excited state (at 1274 KeV) of 22

Ne. The excited state goes to the ground state by the

emission of 1274 –KeV gamma ray with a half life-time η1/2 of 3x10-12

s. Thus positron

emission is almost simultaneous with the emission of 1274 KeV gamma ray while the

positron annihilation is accompanied by two511-KeV gamma rays. The measurements of

47

the time interval between the emission of 1274 KeV gamma ray and 511-KeV gamma

rays can yield the lifetime „η‟ of positron annihilation.

2.4.7 Source Corrections

The amount of positrons annihilating in the source (10-15%) is related to the

geometry of the sandwich configuration, the thickness and density of the foil include, and

of the specific sample for which the measurements are made. The samples enter into the

consideration because the positron may reflect at the sample source interface. These

annihilation contribute additional lifetime components to the lifetime spectrum. In order

to obtain suitable values of lifetimes and their intensities in samples to be studied it is

important to make correction foe these components as precisely as possible. The

correction should be measured using defect free reference samples. Berolaccini and

Zappa [41] have given an empirical formula for the foil intensity as:

I foil (%) = 0.324xZ0.93

xD3.45 / Z 0.44

(2.6)

where Z is the sample atomic number and D is the foil thickness in mg/cm2. At high Z

values (>40) this formula overestimates the foil intensities.

2. 4.8 Commonly used positron-emitting isotopes

Table – 2.1

Isotope Half life β+-

decay(in%)

Maximum

energy or end

point energy

Application in

lifetime

studies

22Na 2.6 Y 90 0.54 Yes

58

Co 71 d 15 0.47 Yes

44

Ti 47 Y 94 1.47 Yes

64

Cu 12.7 h 19 0.66 No

68

Ge 257d 88 1.90 No

57

Ni 36 h 46 0.85 Yes

90

Nb 14.6 h 53 1.50 No

55

Co 18.2 h 77 1.50 Yes

48

2.5 Ultraviolet Visible (UV-Vis.) Spectroscopy

Absorption methods involve determination of the reduction in power suffered by a

beam of radiation as a consequence of passing through the absorbing medium. When an

electromagnetic radiation in UV-Vis region (200-800 nm) falls on the target material, a

part of the incident radiation is absorbed by the atoms leading to the transition of the

orbital shell electrons. Ultraviolet visible (UV-Vis.) spectroscopy is the powerful

analytical tool which gives an idea about the value of optical band-gap energy (Eg) and

thus provides an important tool for investigation. In fact Ultraviolet and visible (UV-Vis.)

absorption spectroscopy is the measurement of the attenuation of a beam of light with

wavelength after it passes through a sample or after reflection from a sample surface. The

short wavelength limit for simple UV-Vis Spectrometers is 180nm due to absorption of

ultraviolet wavelength below 180nm by the atmospheric gases. The absorbance A, is

related to the input and output intensities according to the Beer-Lambert Law [42] which

is shown in equation [2.7]

A

II eo

(2.7)

The absorbance, A, Can be divided by the path length, l to yield absorption

coefficient [45] α which quantities quantifies the absorbance per meter, thus taking film

thickness into account [2.6]

α(λ) = 2.303 l

A (2.8)

The absorption of light energy by polymeric materials in the ultraviolet or visible

radiation region corresponds to excitation of outer electrons. When an atom or molecule

absorbs energy, electrons are promoted from their ground state to a higher energy state

(excited state).

Figure 2.7 depicts this excitation process which is quantized. The electromagnetic

radiation that is absorbed has energy equal to the energy difference between the excited

and ground states. The order of the energy changes are of 125 to 650 kJ/mole.

49

E (excited)

E = [E (excited) – E(ground)] = hν

E (ground)

Figure 2.7: The excitation process

In a molecule, the atoms can rotate and vibrate with respect to each other. These

vibrations and rotations also have discrete energy levels which can be considered as being

packed on the top of each electronic energy level as shown in Figure 2.8. Absorbance of

ultraviolet and visible radiation in molecules is restricted to certain functional groups

(chromophores) that contain valence electrons of low excitation energy. The spectrum of

a molecule containing these chromophores is complex. This is because the superposition

of rotational and vibrational transition on the electronic transitions gives s combination of

overlapping lines. This appears as a continuous band. The various possible electronic

transition in organic molecule are shown in Figure 2.9.

Figure 2.8: Electronic energy level diagram, Eo represents the ground state and E*

is the excited state.

Figure 2.9: Energy level diagram for different electronic transitions

50

The π → π* transition requires lesser energy and hence transition of this type

appears at longer wavelength. These are the transition of interest in the study of ion-beam

induced optical modification in polymers.

Irradiation of polymeric materials results in the shifting of absorption edge from

UV towards the visible region. This shift can be correlated with the optical band gap (Eg)

using Tauc‟s expression [44]

ω2

ε2(λ) = (hω- Eg)2 (2.9)

where ε2(λ) is the imaginary part of the complex refractive index, i.e., the optical

absorbance and λ is the wavelength. Eg is usually derived from the plot ε2(λ) versus 1/λ.

The intersection of the extrapolated spectrum with abscissa yields the gap wavelength

(λg), from which gap energy can be derived by

Eg = g

hc

(2.10)

Further, the compounds having double or triple bonds and phenolate or quinonic

structures favour cluster formation under suitable ion irradiation. The number of carbon

hexagon rings in the cluster „N‟ can be found from the Robertson relation [45].

N

Eg2

eV (2.11)

Here 2β is the band structure energy of a pair of adjacent π sites and its value is taken as -

2.9eV for a six numbered carbon ring. Fink et al have pointed out that the Robertson

equation under estimates the cluster size in irradiated polymers. Thus the structure of the

cluster was assumed to be like a buck minister fullerene, that is, a C60 ring instead of C6

and the relation emerges:

N

Eg3.34

e V (2.12)

where N is the no. of carbon atoms per cluster in the irradiated polymer .Above relation

has been used to calculate obtained the no. of carbon atoms per cluster in the irradiated

samples.

51

2.5.1 Instrumentation

The typical ultraviole–Visible spectrometer consists of a light-source, a

monochromator and a detector as shown in figure 2.10. The light source is usually a

deuterium lamp which emits electromagnetic radiation in the ultraviolet region of the

spectrum. A second light source, tungsten lamp is used for wavelengths in the visible

region of the spectrum. The monochromator is diffraction grating; its role is to spread the

beam of light into its component wavelength. A system of slits focuses the desired

wavelength of the sample cell. The light that passes through the sampler cell reaches the

detector which required the intensity of the transmitted light (I). The detector is generally

a photo multiplier tube, although in modern instruments photodiodes are used.

Figure 2.10: Typical Ultraviolet –Visible spectrometer

In a typical double – beam instrument, the light emanating from the light source is

split into two beams, the sample –beam and the reference beam. When there is no sample

cell in the reference beam, the detected light is taken to be equal to the intensity of light

entering the sample (Io). The spectrum is generally recorded as plot of absorbance versus

wavelength.

52

2.6 X-Ray Diffractions Study (XRD)

X-rays can be used for chemical analysis in three different ways:

The first method uses the fact that X-rays emitted by an excited element have a

wavelength characteristic of that element and the intensity proportional to the number

of excited atoms. The excitation can be caused by direct bombardment of the target

material with electrons (direct emission analysis and electron probe microanalysis) or

by irradiation of material with X-rays of shorter wavelength (fluorescent analysis).

The second method utilizes the different absorption of X-rays by different materials

(absorption analysis).

The third method involves the diffraction of X-rays by crystals having geometrically

periodic arrangement of atoms separated by distance comparable to X-rays

wavelength (diffraction analysis). This method is widely used for qualitative

identification of crystalline phases. The condition for diffraction of a beam of X-rays

from a crystals is governed by the Bragg equation:

2dsin (θ) = nλ with λ 2d (2.13)

where λ is the wavelength of the X-rays, d is the interplanar spacing for a family of

planes; n is the order of the diffraction and θ the incoming diffraction angle.

In thin films, X-rays are diffracted by the oriented crystallites at a particular angle

to satisfy the Bragg‟s condition. Having known the values of and , one can calculate

the interplaner spacing. Schematic view of XRD is shown in0 Figure 2.11

Figure 2.11: A schematic of x-ray diffractometer.

53

The XRD can be taken in various modes such as -2 scan mode, -2 rocking curve,

and scan shown in figure 2.12.

Figure 2.12 An illustration of -scan x-ray diffraction, where, ω – angle between incident x-rays and sample surface, 2θ – angle between incident x-rays and detector, ψ –

sample tilt, φ – in-plane sample rotation, x, y – in-plane displacement of sample, z –

vertical displacement of sample.

In the -2 scan mode, a monochromatic beam of X-ray is incident on the sample

at an angle of with the sample surface. The detector motion is coupled with the x-ray

source in such a way that it always makes an angle 2 with the incident direction of the

X-ray beam. The resulting spectrum is a plot between the intensity recorded by the

detector and 2.Reflection geometry was used in measurements (sample were thin film

samples and other possible geometry is the transmission geometry.

The crystallite size in pristine and irradiated polymers was determined by Scherrer

formula (2.14).

Crystallite size (L) =

cosd

K (2.14)

where K is the shape factor of the average crystallite (0.9), is the wavelength (1.54 Å)

for Cu Kα1, d is full width at half maxima (FWHM) and θ is the peak position in radian.

In the present work, the XRD pattern for the bulk and thin films of different

polymers were recorded at Inter-University Accelerator Centre (IUAC), New Delhi using

D8 Advanced Bruker diffractometer with Cu-K radiation (=1.541838Å) at room

temperature by taking 0.020 step size. The cathode was maintained at 30 kV. Diffraction

patterns were recorded in the range 20o ≤ 2θ ≤ 80

o.

54

2.7 Fourier Transform Infrared (FTIR) Spectroscopy

Infrared spectroscopy is one of the most powerful analytical techniques which

offer the possibility of chemical identification. It involves the twisting, bending, rotating

and vibrational motions of atoms in a molecule. Upon interaction with the IR radiation,

some portion of the incident radiation is absorbed at particular wavelengths. The

multiplicity of vibration occurring simultaneously produces a highly complex absorption

spectrum which is uniquely characteristic of the functional groups comprising the

molecule and of the overall configuration of the atoms as well.

For IR absorption to occur, two major conditions must be fulfilled:

Energy of radiation must coincide with the energy difference between the excited

and the ground state of the molecule. The radiant energy will then be the absorbed

by the molecule, increasing its vibration.

The vibration must entail a change in electrical dipole moment.

The infra-red spectrum of a compound is essentially thebe superposition of

absorption bands of specific functional groups. No two compounds will have same infra-

red spectra (except optical isomers). Thus, infra-red spectra is regarded as the fingerprint

of a molecule. The higher frequency portion of the infra-red spectra (4000-1300 cm-1

) is

called the functional group region which shows the absorption arising from stretching

vibrations and are useful for identification of the functional groups. The absorption

pattern in the region 1400-650 cm-1

is unique for a particular compound and hence called

fingerprint region. Both the stretching and bending modes of vibration give rise to

absorption in this region.

2.8 Electrical Studies

If a polymer contains glacial groups is placed in an electric field, direction of its

units and smaller kinetic units will be observed at a definite field – frequency ratios, and

this gives rise to values: dielectric constant and tan (dissipation factor). Important

method that takes place in any dielectric material under the manipulate of electric field is

polarization i.e. the limited displacement of bound charges or orientation of dipole

55

molecules. The dielectric polarization may be judged in terms of the dielectric constant

and the dissipation factor (loss angle or tan).

The best dielectric materials are those which contain a minimum of charge carriers

and potential charge carriers which may be formed by splitting of covalent, atomic or

molecular bonds under the influence of the energetic ions. The dielectric response of

material provides information about the orientational transnational adjustment of mobile

charges present in the dielectric medium in response to an applied electric field. The most

important property of dielectric materials is ability to be polarized under the action of the

field. The dielectric loss behavior of polymer films is very important because of their

possible applications for insulation isolation and passivity in micro-electronic circuits

[46]. In general polymers are insulators and commonly used in insulation of electric

wires. However, certain classes of polymers have been discovered and used as

semiconductor and capacitors with unusual electrical properties.

The electrical conductivity depends on free ions and not only strongly bonded

with the macromolecules. Therefore, the conductivity of polymers mostly depends on the

presence of low-molecular mass impurities that can serve as source of ions[47]. The

conductivity of the polymers approximately lies between 10-3

to 10-9

ohm-1

cm-1

.

Dielectric constant is mostly used to determine the ability of an insulator to store

electrical energy. The dielectric constant is the ratio of the capacitance induced by two

metallic plates with an insulator between them to the capacitance of the inefficiency of an

insulating material [48]. If the material is to be used for strictly insulating purpose, it

would be better to have a lower dielectric constant.

The capacitance (C) and the dielectric loss (tanδ) measurements have been made

with the help of variable frequency LCR meter (Hewlett Packard 4284 A) in the

frequency range of 1-1000kHz at room temperature. The measured values of capacitance

then have been converted into the dielectric constant (ε) by using the formula:

A

Cd

o (2.15)

56

where d is the thickness of polymer film, A is the area of the electrode plates and εo is the

permittivity of free space. AC conductivity is calculated by the relation given below:

ζa.c = 2πƒtanδεoεr (2.16)

57

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